Interesting results from a mix of 27s with AGMs!
Tried out the mixed bank in a backyard test to see if it was ok to go camping with and not have to come home early.
I have no idea what the bank is in AH with the strange way the AGMs act now, but I chose to run the bank down at 25.5 amps because that is the max using all the lights. 30 amps would be for 600AH, eg. I have 650 as rated total, but can't get that with the "broken" AGMs.
Loaded voltage started at about 12.7 and after five hours it was 12.1, with 127AH drawn. If this were a normal 20 hr rate draw down, the 12.1 loaded would be about right for being at 75% SOC (as seen on previous tests) and the five hours is right. SG of the 27s was 1.265 (was 1.275 at start)
Turned off all lights , no load, immediate bounce back 12.45 so ran a kettle test. 90 amps starting at 12.4-drop to 11.7, was 11.5 when kettle clicked off, bounce back 12.3.
Back to the 25 amps of lights test 12.1 loaded another two hours and voltage 11.9, SG of 27s is 1.200ish (which is 50% SOC)
Wanted to see if it could still do the kettle, and it did, holding at 11.3 before click off-- no crash.
Looking dubious at only 7 hours (instead of 10) to get to 50%, and AH 216, so is "full" only 432? Seems very low.
Disconnected all batteries to take individual bounce back voltages, where 12.4 would be 50% of AGMs and 12.2 for the 27s.
After one hour, the two 27s were at 12.17 and 12.21 so really at 50% confirmed by matching SG numbers.
The two 100AH AGMs (tested in April at 90) were 12.78 for one and 12.69 the other. The 250 was 12.59.
So the AGMs were letting the 27s do the work! But without the 27s added in, no kettle.
So the AGMs were about 75% (by voltage--could be anything actually) while the 27s were at 50% at the end of the test run. Still plenty of camping AH left to go in the AGMs, but the 27s are at 50. If I went further I would still be fine for running things, but the 27s would be taken down below 50 to unknown level.
Puzzle. I need a way to disconnect the 27s and continue (but without high amp draws) just on the AGMs. But how to tell when the 27s are at 50 using the whole bank, unless I take SGs? (No thanks).
I still have the switches where the two AGM 100s can be separated from the 250 so I can recharge them separately, and now that means I can have the 27s and the shaky 250 in one set and the two not as shaky 100s in the other.
One approach might be to start camping with only the 27s and 250 and wait till voltage indicates the 27s are at 50%, then switch so all draws are from the two full 100s, but no high draws allowed. That would give longer till a generator recharge is needed, and keep the 27s above 50%. Total time would be less than using the whole bank at once (Peukert), but gets more out of whatever the AGMs have in them for low amp draws like the furnace and usual RV loads.
Not ideal, but we could probably get by like that until I get new batteries eventually. Have to go camping and find out, now it looks like it ought to work.
